A hydraulic control system of an automatic power transmission for an automotive vehicle includes, in addition to a pressure regulator for producing a main control or line pressure, a transmission throttle valve which is adapted to produce a throttle pressure continuously variable with load on the entine with which the transmission is used together in an automotive vehicle. Among the transmission throttle valves presently in practical use, there is a throttle valve which is responsive to the movement of the accelerator pedal of a vehicle for producing a throttle pressure variable with the degree to which the throttle valve provided in the carburetor of the engine is open. A transmission throttle valve of this nature usually comprises a valve plunger connected to or engaged by the accelerator pedal through a mechanical linkage provided therebetween and a valve spool arranged in series with the valve plunger and engageable with the valve plunger through a spring provided between the valve spool and plunger. In the presence of a line pressure in one fluid inlet port of the throttle valve, the valve spool is not only subjected at one end thereof to the force of the spring but acted upon at the other end thereof by a fluid pressure which is developed by the valve spool attempting to attain a certain equilibrium position in which the force resulting from the fluid pressure urging the valve spool against the force of the spring is substantially equally balanced with the force of the spring. The mechanical linkage provided between the valve plunger and the accelerator pedal is arranged so that the valve plunger is continuously movable with the accelerator pedal. As the accelerator pedal is depressed from the released position thereof, the valve plunger is axially moved toward the valve spool and compress the spring provided between the valve spool and plunger. The force of the spring exerted on the valve spool is thus continuously variable with the distance of movement of the accelerator pedal and is therefore representative of the opening degree of the carburetor throttle valve. Thus, the throttle pressure developed by the transmission throttle valve is a function of the opening degree of the carburetor throttle valve and is variable as the accelerator pedal is moved between the released position and a fully depressed or kickdown position thereof.
When the accelerator pedal is in a condition being depressed by the vehicle driver, the force of the spring intervening between the valve plunger and valve spool of the transmission throttle valve is transmitted to the accelerator pedal through the valve plunger and the mechanical linkage between the valve plunger and the accelerator pedal. The force being applied to the accelerator pedal by a vehicle driver is therefore opposed by the force thus carried over to the accelerator pedal from the transmission throttle valve so that the vehicle driver must depressed the accelerator pedal against such an opposing force. The force opposing the driver's effort to depress the accelerator pedal increases as the accelerator pedal is depressed deeper because the force of the spring between the valve plunger and spool of the transmission throttle valve becomes larger as the valve plunger is moved toward the valve spool.
With a view to providing a solution to this problem encountered in a transmission throttle valve of the described type, it has been proposed that the valve plunger of the throttle valve be provided with a laterally or radially enlarged axial portion forming a pressure acting area to be acted upon by a fluid pressure for urging the valve plunger to move toward the valve spool against the force of the spring between the valve plunger and spool, viz., in a direction in which the valve plunger is to be axially moved when the accelerator pedal is depressed by a vehicle driver. Formation of such an axial portion inevitably gives rise to an increase in the axial length of the valve spool and results in the more elongation of the whole construction of the transmission throttle valve which tends to be elongated because of the valve plunger and spool which are arranged in series.
In order, furthermore, that the pressure acting area thus formed on the valve plunger be constantly acted upon by a fluid pressure independently of the relative movement between the valve plunger and spool, a tubular member must be provided through which the valve plunger is to be axially moved back and forth when the accelerator pedal is depressed and released. Provision of such an additional member arises not only a problem in that the member adds to the number of the component parts of the transmission throttle valve but a problem in that, since the tubular member is also subjected to the fluid pressure acting on the enlarged axial portion of the valve plunger, further additional means are required to prevent the tubular member from being dislodged from a predetermined axial position relative to the valve body structure supporting the valve plunger. All these arrangements are responsible for the complicated and costly construction of the transmission throttle valve of the described type.
The present invention contemplates provision of an improved transmission throttle valve which is sufficiently small in axial length and simple in construction and which is nevertheless reliably operable for alleviating the resistance which a vehicle driver encounters when in depressing the accelerator pedal in an automotive vehicle using an automatic power transmission including a transmission throttle valve of the described basic nature.